This invention relates to a bearing device in which a rolling bearing is fitted around an outer periphery of a shaft body such as a hub unit for vehicles and a method of manufacturing the same.
A hub unit for vehicles generally has a structure wherein a double row rolling bearing is fitted around an outer periphery of a shaft body of a hub wheel so as not to come off.
The shaft body of the hub wheel is provided in a free end side thereof with a cylindrical portion used for the bearing not to come off. The cylindrical portion is bent and deformed radially outward by a caulking jig, and thereby being caulked on an outer end face of an inner ring of the bearing so as to constitute a caulked portion. The bearing is prevented from coming off the hub wheel by the caulked portion, and simultaneously, the inner ring of the bearing is preloaded from the caulked portion.
For the hub unit described above, cracks which may adversely influence the performance of the hub unit are occasionally generated on the surface of the caulked portion. Therefore, an inspection for a crack generation ratio is generally executed to all of hub units which have been manufactured.
In this case, since cracks per se are minute, the inspection using an inspection machine requires a microscopic and laborious work. Furthermore, in the case of a conventional hub unit, a crack generation ratio comes with a scattering result, thereby the crack generation ratio inspection is highly time-consuming, which may result in lowering the manufacturing efficiency of the hub unit.
Therefore, a main object of the present invention is to provide a bearing device capable of accurately controlling a crack generation ratio and a method of manufacturing such bearing device.
Another object of the present invention is to provide a bearing device capable of restraining a crack generation ratio to less than a predetermined value and a manufacturing method thereof.
Still another object, features and advantages of the present invention will be apparent from the described hereafter.
A bearing device of the present invention comprises a shaft body having a caulking cylindrical portion which is formed by a machine work in a free end side of the shaft body and a rolling bearing fitted around an outer periphery of the shaft body. The cylindrical portion is bent and deformed radially outward to be caulked on an outer end face of an inner ring of the rolling bearing. Also, a residual stress is left by the machine work on a surface layer of an inner peripheral surface of the inner ring. The residual stress is set to a value capable of restraining a crack generation ratio to less than a predetermined rate.
A shape of the cylindrical portion comprises all kinds of shapes such as a radial wall thickness thereof is uniform, becomes gradually thinner or varies stepwise intermediately toward the free end side of the shaft body.
The residual stress is not restricted to a strain stress, however comprises a compression stress as well.
The cracks, regardless of a size thereof, are not restricted to cracks generated on the surface of the caulked cylindrical portion, however comprises all forms of cracks such as cracks spreading inside.
A value less than the predetermined value may or may not include zero.
The machine work comprises a turning work using a turning tool, a drilling work using a drill and other works using other machine tools.
The formation of the cylindrical portion on the free end side of the shaft body using a machine tool accompanies heat generation. When the heat temperature becomes too high and the cylindrical portion is overheated thereby, a texture of a surface layer of an inner peripheral surface thereof is degenerated into slightly stiffening or the like. A stress is likely to be left in the degenerated surface layer of the inner peripheral surface of the cylindrical portion. Depending on the amount of the residual stress, cracks are easily generated during the process of radially-outward bending of the cylindrical portion.
In the case of the bearing device according to the present invention, the residual stress is set according to such a manner of the crack generation. This enables restraining the crack generation ratio to less than the predetermined rate.
Therefore, according to the bearing device of the present invention, it is possible to provide the bearing device wherein the crack generation ratio is less than the predetermined rate on the basis of control of the residual stress in the cylindrical portion and by restraining the crack generation ratio. As a result, an inspection for the crack generation ratio becomes unnecessary, leading thereby to an improved manufacturing efficiency.
As a preferable embodiment of the present invention, the cylindrical portion is formed by a turning work.
As a preferable embodiment of the present invention, the cylindrical portion is bent and deformed radially outward by a rolling caulking.
As a preferable embodiment of the present invention, the residual stress is set to a value of 5 kgf/mm2 or less.
As a preferable embodiment of the present invention, the residual stress is set to a value of 10 kgf/mm2 or less.